1. Field of Invention
This invention relates to the field of the radio telephone apparatus such as cellular radio telephones and cordless telephones and, more particularly, to an improved portable radio telephone apparatus couplable to an adapter unit enabling hand-free operations.
2. Description of the Relevant Art
As cellular radio telephones come into wide use, various types of vehicle adapters for the radio telephone have been developed. For example, U.S. Pat. No. 5,048,117 to Aisaka et al. discloses an adaptor unit which may amplify radio frequency signals output from a portable radio telephone in response to a level control signal. As another example of the vehicle adaptor, commonly assigned U.S. Pat. No. 5,045,115 to Scicoc, et al. 07/249,817, filed Sep. 27, 1991 entitled "Radio Telephone Apparatus" discloses a radio telephone apparatus wherein a portable radio device may be connected to an additional unit having a speaker and a microphone via a cable, so that hand-free operations becomes available to the user of the apparatus when they are connected. By means of these types of adaptor, a user may have a telephone conversation with hand-free operations.
According to such conventional adapters, however, the user has to first connect the portable radio unit with the adaptor unit by the cable and turn on both units in order to enjoy the hand-free conversation. The adapter may be kept on regardless of the connection relationship between the portable radio unit and the adapter unit. In such a case, electric power is wastefully consumed in the adapter unit when the adapter unit is not connected to the portable radio unit.